A number of systems have been proposed for utilizing gaseous and liquid fuels in internal combustion engines, but many devices have involved complex changes to the engine with complex controls and switches which are not user friendly. One device disclosed in U.S. Pat. No. 4,068,639, suggests the use of a gaseous fuel reservoir and manual switching controls. U.S. Pat. No. 3,753,424 injects a gaseous fuel only at load conditions of the engine and not at a constant pressure as in the present invention. U.S. Pat. Nos. 3,718,000, 2,381,304, and 3,659,574 use either a gaseous fuel or a liquid fuel but never both at the same time.
The system disclosed in U.S. Pat. No. 4,373,493 utilizes a gaseous fuel injected into a liquid fuel carburetor. It varies the amount of gaseous fuel flow with engine load and injects an additional amount of atmospheric air mixed into the gaseous fuel port. This system requires shutting off the supply of liquid fuel at idle and requires the engine to operate at idle on only the gaseous fuel, unlike the present invention which only substitutes a portion of the liquid fuel with gaseous fuel at idle.
Consequently if an air conditioner or other intermittent devices such as fans, radios, lights, etc., are operated at idle the engine may stall or the idle has to be set higher than normal to prevent stalling, thus decreasing fuel efficiency.
The system disclosed in U.S. Pat. No. 4,373,493 being open to the atmosphere permits leakage of gaseous fuel to the atmosphere.